in photoshop pick a gray tone in the grayscale mode. then convert it to a bitmap. then you cut out the area you want and place it on your b/w art and multiply that layer. you'll have to play around with it to really see all your options. and you can also invert it so you have white dots on black. and you can do a gradient in the half tone and convert that to a bitmap to get a zipatone fade. like i said- there's lots of options- just got to play around with it. hope this makes sense.